Hi,

I have a Schwinn Traveler with a bent fork. I am wondering if I can buy cheap used Schwinn (probably like a single speed) off Craigslist and use its fork. (And maybe is fenders and brakes too). I wouldn't know how to measure the sizes. The wheels are 27" but I don't know about forks.

Sorry I am a novice. Thanks.

One hint is the wrong forks won't be long enough, so you're likely confined to another bike with 27" wheels.
Most anything that old should work as long as the steerer tube is long enough.
A "large" bike should have a longer steerer than a small bike.

...replacing a fork (even one on a bike that old), is not something I would recommend that a novice do by him or herself. If there's a bike co-op near you, go there to look for used forks and some assistance. In swapping it out.

If you're extremely lucky, you might find someone who knows how and can straighten your existing steel fork. They're not all salvageable, but some of them are if not too badly bent.